1. Field of the Invention
The present invention relates to television signal processing apparatus and, more particularly, to luminance signal processing apparatus including rise time normalization and high frequency noise attenuation.
2. Background of the Disclosure
A well known technique for reducing the low level background noise in video signals is to selectively remove (or at least attenuate) high frequency, low amplitude signal components. One type of television receiver luminance processor, the removal is accomplished by subtracting a fixed offset from all levels of the sharpness or peaking signal. The transfer characteristic of an amplifier used for this purpose in the known processor is illustrated in FIG. 1. In that figure, solid line a is a plot of peaking signal as a function of received signal. Dashed line b runs parallel to line a and indicates the same transfer characteristic, but without the fixed offset. Vertical line a indicates the magnitude of offset, or, in other words, the amount by which the viewer must increase the sharpness of peaking signal on mid-level transitions in order to achieve the same sharpness as he would have had with a linear transfer without offset. Line d shows the transfer characteristic at this increased level.
It will be noted that this characteristic results in excess peaking at high level transitions as indicated by vertical line e. This overpeaking is undesirable.
A further, related problem results from the fact that, in presently known luminance processors, the user-selected peaking level discussed above is maintained by monitoring the maximum amplitude of the first or second derivative of the peaked luminance signal over a whole scene, and then holding this quasi-DC level constant by means of negative feedback to a gain controlled amplifier. This method works well on scenes which contain the same magnitude of maximum luminance step, e.g. test patterns. It does not yield acceptable results for the usual video picture material. If the sharpness control is initially set for a pleasing picture on "average" scene material, screens having full transitions tend to be under peaked, while those with minimal transitions (e.g. a fade to black) tend to be overpeaked and noisy.
Finally, it was found that in a known television receiver, the coring and auto-sharpness circuits described above are cascade-connected with the coring circuit preceding the auto-sharpness circuit. Resulting waveforms for the case of a normal incoming vides signal and a smeary incoming video signal are shown on the left-hand side of FIG. 2, and on the right-hand side of FIG. 2, respectively. It will be noted that the smeary picture would have a peaking signal (line b) of lower amplitude. The waveforms after coring in the respective cases are shown in line c. It will be noted that the smeary incoming signal results in a relatively large zero output peaking signal between the positive and negative half cycles, since a fixed amplitude core has been taken out. As illustrated in line d, the output of the known auto-sharpness circuit receiving the known coring signal illustrated in line c would have different effective coring widths depending upon the type of signal received. The coring width for the smeary signal may well be too wide to be acceptable.